Appearance
Gun drilling is the original deep hole drilling method — developed in the 19th century for rifle barrels, it remains the go-to process for small-diameter, high-precision holes with L/D ratios exceeding 100:1.
How Gun Drilling Works
A gun drill consists of three sections: the carbide tip (cutting portion), the steel shank (a tube with a V-shaped flute rolled or milled into it), and the driver (the cylindrical end held by the machine spindle).
High-pressure coolant is pumped through the hollow shank, exits at the cutting face, and returns along the external V-groove, carrying chips out of the hole. The tool cuts eccentrically — only one cutting edge engages the workpiece — and two guide pads on the carbide tip ride against the bore wall, providing self-piloting guidance.
Gun drilling does not require pecking
Unlike conventional twist drilling, gun drilling runs continuously from entry to exit. The high-pressure coolant stream ensures continuous chip evacuation, making the process both faster and safer for deep holes.
Single-Lip Cutting Action
The single-lip design creates an unbalanced radial force that pushes the guide pads firmly against the bore wall. This is deliberate — the resulting burnishing action improves surface finish and straightness. The cutting edge is offset from the drill center, leaving a small triangular or rounded center section that acts as a pilot.
The most critical element of the tool is the nose grind — the geometry of the cutting face. Different grind types suit different materials:
| Grind Type | Best For |
|---|---|
| N-8 (General purpose) | Steel, Inconel, stainless steel |
| N-4 | Aluminum, brass, soft non-ferrous |
| N-73 | Angular entries, stacked parts (strongest point) |
| Facet / F8 | European standard, enhanced coolant clearance |
| Copper grind | Near-flat-bottom applications |
Key Parameters
Cutting Speed by Material
| Material | Cutting Speed (m/min) |
|---|---|
| Aluminum | 80 – 160 |
| Brass | 80 – 150 |
| Carbon & alloy steels | 70 – 100 |
| 42CrMo4 / 50CrMo4 | 70 – 80 |
| Cast iron (gray, ductile) | 70 – 100 |
| Stainless steel | 50 – 80 |
| Titanium alloys | 30 – 60 |
| Inconel / superalloys | 15 – 40 |
Feed Rate by Diameter
| Diameter (mm) | Steel (mm/rev) | Cast Iron (mm/rev) |
|---|---|---|
| 3 – 4 | 0.007 – 0.013 | 0.009 – 0.038 |
| 5 – 6 | 0.016 – 0.026 | 0.018 – 0.068 |
| 8 – 9 | 0.024 – 0.036 | 0.032 – 0.096 |
| 10 – 12 | 0.030 – 0.049 | 0.050 – 0.120 |
| 16 – 18 | 0.048 – 0.079 | 0.079 – 0.170 |
| 20 – 24 | 0.060 – 0.107 | 0.106 – 0.207 |
| 32 – 40 | 0.085 – 0.154 | 0.160 – 0.245 |
WARNING
These are starting ranges. Actual parameters depend on machine rigidity, coolant system capacity, and specific material condition. Always consult the tool manufacturer's recommendations for your specific application.
Coolant Pressure Requirements
Coolant pressure is the single most critical process parameter in gun drilling. Insufficient pressure leads to chip jamming, tool breakage, and scrap.
| Diameter (mm) | Minimum Pressure (bar) | Recommended Pressure (bar) |
|---|---|---|
| 2 – 4 | 80 | 100 – 150 |
| 5 – 8 | 50 | 80 – 120 |
| 10 – 15 | 30 | 60 – 100 |
| 16 – 25 | 20 | 40 – 80 |
| 25 – 50 | 15 | 30 – 60 |
Filtration is equally important — coolant should be filtered to 10–20 µm to prevent nozzle blockage and surface finish degradation.
Guide Pads: The Self-Piloting Mechanism
Guide pads are the unsung heroes of gun drilling. These carbide strips, mounted on the drill head, serve two functions:
- Guidance — They bear against the machined bore wall, centering the tool and maintaining straightness
- Burnishing — The pad pressure smooths the bore surface, improving finish by up to 70% compared to the cutting action alone
Guide Pad Troubleshooting
| Problem | Likely Cause | Correction |
|---|---|---|
| Rapid pad wear | Speed too high, insufficient coolant | Reduce speed, increase pressure |
| Scoring on bore surface | Chips trapped between pad and wall | Improve filtration, check coolant flow |
| Oversized hole | Worn pads | Regrind or replace tool |
| Vibration / chatter | Incorrect pad geometry | Consult manufacturer for material-specific grind |
| Seizing in hole | Insufficient back taper | Increase back taper on tip |
Industrial Applications
| Industry | Typical Components | Diameter Range | L/D Ratio |
|---|---|---|---|
| Automotive | Crankshafts, camshafts, fuel injectors, fuel rails | 1 – 15 mm | 20:1 – 100:1 |
| Aerospace | Landing gear, turbine shafts, structural components | 3 – 30 mm | 30:1 – 150:1 |
| Medical | Bone screws, intramedullary nails, surgical instruments | 0.5 – 6 mm | 10:1 – 80:1 |
| Hydraulics | Valve bodies, spool bores, cylinder components | 2 – 25 mm | 20:1 – 100:1 |
| Mold & die | Cooling channels, ejector pin holes | 3 – 15 mm | 30:1 – 150:1 |
| Defense | Firearm barrels, breech components | 5 – 30 mm | 50:1 – 200:1 |
| Energy | Fuel injector nozzles, heat exchanger tubes | 1 – 20 mm | 20:1 – 100:1 |
Advantages and Limitations
Advantages:
- Extremely high depth-to-diameter ratios (up to 200:1 standard, 500:1 with specialized equipment)
- Excellent straightness — 0.1–0.5 mm per meter
- Good surface finish in a single pass (Ra 0.4–1.6 µm)
- No pecking required — continuous feed from entry to exit
- Tolerance capability of IT7–IT10
- Works on conventional CNC machines with appropriate coolant systems
Limitations:
- Lower material removal rates compared to BTA drilling (5–7× slower feed)
- External chip evacuation can score the bore surface in soft materials
- Limited to diameters up to approximately 50 mm
- High coolant pressure requirements add system cost
- Tool geometry is material-specific — not a one-size-fits-all process
FAQ
What is the difference between gun drilling and conventional drilling?
Gun drilling uses a single-lip, single-flute tool with internal coolant delivery and external chip evacuation. The tool is self-piloting via guide pads. Conventional twist drilling uses a two-flute symmetrical tool that relies on the drill point geometry for centering. Gun drilling achieves much higher L/D ratios (100:1+ vs ~10:1) and better straightness without pecking.
Can gun drilling be done on a standard CNC mill or lathe?
Yes — with a high-pressure coolant system (minimum 40 bar, ideally 80+ bar) and a whip guide for support. Ejector drilling was specifically designed for retrofits, but gun drilling is also practiced on conventional CNC equipment. The main requirements are through-spindle coolant capability and adequate coolant filtration (10–20 µm).
What coolant pressure is needed for gun drilling?
Minimum 30 bar for diameters above 15 mm, increasing to 100–150 bar for diameters below 5 mm. Pressure requirements scale inversely with diameter — smaller holes need higher pressure to overcome flow resistance and maintain adequate velocity for chip evacuation.
How long does a gun drill last?
Tool life varies widely by application. In production environments, a single gun drill may produce 500–3000 holes before requiring regrinding. With proper regrinding (typically 3–5 times before replacement), total tool life can reach 10,000+ holes. The limiting factor is usually guide pad wear or carbide tip chipping rather than catastrophic failure.
How straight is a gun-drilled hole?
Standard gun drilling achieves straightness of 0.1–0.5 mm per meter of depth. With counter-rotation (workpiece and tool rotating in opposite directions), straightness can reach 0.02–0.05 mm/m. Primary factors affecting straightness include: guide bushing fit, guide pad condition, feed rate consistency, and material homogeneity.
Parameters are starting recommendations. Actual values depend on machine condition, coolant system, workpiece material, and specific tooling geometry. Always consult your tool supplier for application-specific data.